Actually I misspoke. It will automatically upload, but it shouldn't transfer the file to the destination directory. It would remain in the tmp directory and be removed later. Is the file being uploaded to the destination directory? It looks like it would work to me.
Actually I think I see it. The value you should be using should be from the FORM bindings, not the UploadFileField bindings. That wouldn't be available until after the webassist upload code runs.